WHEN the young Charles Darwin set off round the world on HMS Beagle in 1831, he took with him only those books he felt essential. The ship was small, his cabin cramped and Darwin had to limit his library to what would fit on one small shelf.

He chose the Bible and a few key works on botany, zoology and geology. Then, after asking special permission from the captain, he added Alexander von Humboldt’s Personal Narrative, a weighty seven-volume account of the Prussian naturalist’s travels in South America 30 years earlier.

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Humboldt was Darwin’s hero: his vivid descriptions of the tropics and revolutionary ideas about nature were the reason Darwin signed up for the Beagle.

Historian Andrea Wulf calls Humboldt the lost hero of science. It is extraordinary that a man once so revered is now largely forgotten, the more so given his name is dotted over maps of half the world, attached to mountains, lakes, rivers and towns – and even to a “sea” on the moon. Humboldt is also the name of an ocean current, a penguin and a long list of other animals and plants.

During his long life, however, the naturalist, aristocrat-turned-revolutionary and writer of some of the most influential books of the 19th century was a guiding light to generations of scientists, poets and politicians who are now far more celebrated than he is. For a while, he had the world at his feet, and was considered the most famous man after Napoleon.

In this gripping study, Wulf follows Humboldt from his stifling youth in Prussia to the liberating rainforests of South America, where, during an arduous five-year expedition, he dodged crocodiles on the Orinoco and fell asleep to the sound of snoring river dolphins.

On the flooded grasslands of Venezuela’s Los Llanos, he discovered electric eels and tested their potentially lethal power. In the Andes, despite an injured foot, he climbed higher than anyone before as he tried to reach the peak of Chimborazo, a volcano then thought to be the world’s highest mountain. And all because he believed that to understand nature you couldn’t just study it but had to experience it.

A drawing of a New World monkey in Humboldt’s Personal Narrative (Image: BPK/Staatsbibliothek zu Berlin)

From the beginning, Humboldt approached the natural world in a different way to most scholars. While they collected, named and classified, he looked at the bigger picture. He saw nature in terms of what we now call ecosystems. Life wasn’t a matter of individual plants and animals but a web in which everything was linked. As he wrote: “In this great chain of causes and effects, no single fact can be considered in isolation.” Break one link and the whole would fall into disarray.

But taking a broad view didn’t mean ignoring the detail. Far from it. Humboldt was obsessed with data. He had instruments to measure everything, even the blueness of the sky, lugging them from one side of South America to the other.

Wherever he went, he recognised patterns, spotting similarities in the types of plants growing in widely separated parts of the world, thereby identifying the vegetation zones that girdle the globe. And climbing in the high Andes, he saw stretched out below the same phenomenon, with altitude rather than latitude dictating what grew where.

His travels also opened his eyes to the horrors of colonialism and the damage done to both people – he was a fierce opponent of slavery – and the environment. Land cleared to grow cash crops for Spanish landowners, wrote Humboldt, soon became dry and barren. He recognised the role forests play in controlling local climate – and how the loss of trees would change it.

Years later, Humboldt wrote about how people were changing the environment, and listed three ways in which humans were affecting climate: deforestation, ruthless irrigation, and the “great masses of steam and gas” produced in the world’s industrial centres. As Wulf suggests, Humboldt was the unrecognised great-grandfather of environmentalism.

“Humboldt was the unrecognised great-grandfather of environmentalism”

But the key to his fame was undoubtedly the way he wrote. Immersed in culture from his youth, his social circle included artists and poets, among them the German poet Goethe. The result, says Wulf, was that Humboldt the scientist wrote like a poet. French writer François-René de Chateaubriand thought his writing so evocative that “you believe you are surfing the waves with him, losing yourself with him in the depths of the woods”.

Humboldt’s accounts of his travels were translated, pirated and sold worldwide, and they won him legions of admirers. Poets mined his writing for ideas; Simon Bolivar, liberator of Spain’s South American colonies, saw his homeland through Humboldt’s eyes and was inspired to start a revolution. “With his pen”, said Bolivar, Humboldt had awakened South America. And aboard the Beagle, Darwin read and re-read Humboldt’s account of the tropics, turning over the great man’s ideas while he formulated his own.

Wulf has delved deep into her hero’s life and travelled widely to feel nature as he felt it. “Humboldt gave us our concept of nature itself,” says Wulf. “The irony is that Humboldt’s views have become so self-evident that we have largely forgotten the man behind them.” No one who reads this brilliant book is likely to forget Humboldt.